Brood stocks

The article is based on an ongoing collaboration in Vietnam between the WorldFish Center (formerly ICLARM) and the Ministry of Fisheries, at the Research Institute for Aquaculture No. 3, Nha Trang City, Khanh Hoa Province. The work described is oriented towards regions near the equator, where induced spawning of on-grown broodstock should be possible over about 10 months of the year. A shorter breeding season (in subtropical areas) would necessitate bigger installations, but a larger market size would have the opposite effect.

Between May and October 1990, fecundity, egg size and condition factor of Chrysichthys nigrodigitatus (Lacépède) in the Cross River, Nigeria, were studied. The fecundity (F) of this population varied from 3 046 eggs (total length, L=28.5 cm) to 28 086 eggs (L=64 cm). A mean relative fecundity of 231 eggs/cm or 13 eggs/g of fish was obtained for this population. The fecundity of this population can be estimated with the formula F=2.511 · L 2.30 or F=52.893 · W 0.78 , total length being in cm and weight (W) in g. The mean egg diameter of this population varied from 0.65 mm to 3.54 mm.

Following a survey of the important traits of Indian carp broodstock at some southern Indian hatcheries, it was found that the broodstock selection was size selective, exerting strong, negative selection of prematuration growth rate and positive selection on age at first maturation. This meant that the hatchery bred inadvertently slower growing and later maturing individuals. Details are given of approaches to avoid such negative selection and minimize inbreeding.

Effects of presence/absence of a spawning substrate, and of temperature and water quality changes on seasonal fry production by Tilapia rendalli broodstock, were investigated from 29 December 1989 to 19 February 1991 (416 days) in tanks at the National Aquaculture Center, Domasi, Malawi. For each of the two treatments (with vs. without substrate), four 5-m super(3) tanks were stocked with T. rendalli broodstock of similar mean size (35.8-44.2 g). Fish produced batches of sticky eggs which adhered strongly to side and bottom walls of cement tanks.

Based on the encouraging results obtained by earlier workers, the concept for a floating hatchery was developed for producing tilapia for both farming and enhanced fisheries in the freshwater lakes and coastal lagoons of Gabon. The research and development work to test this concept was undertaken with Nile tilapia (Oreochromis niloticus). Two places in Gabon were selected, representing climatic and environmental condition similar to other parts of the country.

The freshwater river systems and floodplains of Bangladesh are the breeding grounds for 13 endemic species of carps and barbs and a large number of other fish species, including a number of exotic carps and other species that have been introduced for aquaculture. Since 1967, breeding of endemic and exotic aquaculture species for seed producton through hypophysation techniques has become a common practice.

The controlled breeding of finfish for culture is reviewed with special refer-ence to recent developments and persistent problems. Freshwater species are at present cultured on a much larger scale than brackishwater and marine species which have potential for aquaculture in arid and semi-arid lands. A reliable supply of fish seed for freshwater farming can usually be produced from captive broodstock whereas coastal aquaculture still depends largely on collection of seed from the wild.

The International Network on Genetics in Aquaculture (INGA) was established in 1993 and is being coordinated by the International Center for Living Aquatic Resources Management (ICLARM), with the objective to contribute through collaborative research, to the domestication and sustainable performance of tropical finfish species farmed in developing countries and to strengthen national capabilities for genetic enhancement of farmed fish through exchange of germplasm, methodologies and through training and interactive forums.

Oreochromis shiranus broodstock were stocked in 3 x 3 x 1 hapas for 413 days in Malawi at 1.7, 1.0, and 0.7/m super(2) (sex ratio was 1 male:2 females). First sampling was done 14 days after stocking and every 21 days thereafter. Free-swimming fry first appeared after 56 days. Total spawn (free swimming fry, sac fry, and eggs) were harvested every 21 days, beginning on day 224. The lowest broodstock density produced significantly (P < 0.05) more free-swimming fry and spawn per female, but the highest density produced significantly more per m super(2).

Human and institutional capacities for developing and managing genetically improved tilapia in Africa are discussed. Discussions are related particularly to the status of hatcheries, rearing facilities, research and extension services, training in genetic enhancement, and fish transfer in major aquaculture countries in Africa. The leading aquaculture producing countries are Egypt and Nigeria along with nine other countries with some intermediate levels of fi sh production. The availability of quality fry and fingerlings constitutes a major constraint.